The catalytic cracking of hydrocarbon oils using acidic cracking catalysts is a well established process which has, for a number of years, used a number of different types of catalytic cracking units including, in the early years, fixed bed crackers of the Houdriflow type and later, moving bed units such as the Thermofor Catalytic Cracking (TCC) units and fluidized bed catalytic cracking units (FCC). Of these, fluid catalytic cracking (FCC) has now become the predominant type of unit for catalytic cracking. In both the moving, gravitating bed and moving, fluidized bed processes, the feedstock to the unit is brought into contact with a hot, continuously circulating, cracking catalyst to effect the desired cracking reactions, after which the cracking products are separated from the catalyst which is regenerated by oxidation of the coke which accummulates on the catalyst. Oxidative regeneration in this way serves the purpose both of removing the coke which deactivates the catalyst and also brings the catalyst back up to the temperature required to maintain the endothermic cracking reactions. The hot, regenerated catalyst is then recirculated to the reactor where it is again brought into contact with the feedstock. The moving bed (TCC) process, the catalyst is generally in the form of beads which move through the reactor and the regenerator in a solid, gravitating mass whereas in the FCC process, the catalyst is in the form of a fluent powder, typically of about 100 microns particle size.
The catalysts used in catalytic cracking, whatever the type of unit employed, possess acidic functionality in order to catalyze the cracking reactions which occur. Initially, the acidic functionality was provided by amorphous type catalysts such as alumina, silica-alumina or various acidic clays. A significant improvement in the process was provided by the introduction of crystalline, zeolitic cracking catalysts in the 1960's and this type of catalyst has now become universally employed. The zeolites which are used for this purpose can generally be characterized as large pore zeolites because it is essential that the internal pore structure of the zeolite which contains the bulk of the acidic sites on the zeolite should be accessible to the bulky, polycyclic aromatic materials which make up a large portion of the heavy oil feeds to the process. Large pore zeolites which have been used for this purpose include mordenite and the synthetic faujasite zeolites X and Y. Of these, zeolite Y has now become the zeolite of choice because of its superior stability to hydrothermal degradation, particularly when it is used in the forms of a rare earth exchanged zeolite (REY) or the so-called ultrastable Y (USY).
Although most of the feeds to catalytic cracking units contain significant amounts of high boiling aromatic constituents, some feeds, particularly from Southeast Asian and Pacific sources contain relatively large amounts of waxy paraffins which are relatively refractory towards catalytic cracking, especially in the presence of aromatics. Feedstocks of this type are generally difficult to process in conventional catalytic cracking processes regardless of the type of catalyst used: when waxy gas oils derived from crudes of this type are passed through the unit, the gasoline product tends to have a relatively low octane number and the unconverted fraction in which the refractory paraffins tend to concentrate, has a very high pour point which makes it unsuitable for use as a blending component in fuel oils without the addition of cutter stock. Furthermore, recycle of the unconverted fraction is of limited utility because of the refractory nature of the paraffins in this material.
The problems presented by the presence of waxy components in petroleum oils have, of course, been known for a long time and various processes have been evolved for removing the waxy components from various distillate fractions including lubricating oils, middle distillates including heating oils and jet fuels and gas oils. Various catalytic hydro-dewaxing processes have been developed for this purpose and these processes have generally removed the longer chain n-paraffins and slightly branched chain paraffins by selectively cracking these materials to produce lower molecular weight products which may be removed by distillation. In order to obtain the desired selectivity, the catalyst has usually been an intermediate pore size zeolite with pore size which admits the straight chain n-paraffins either alone or with only slightly branched chain paraffins, but which excludes more highly branched materials, naphthenes and aromatics. Catalytic hydro-dewaxing processes of this kind are described, for example, in U.S. Pat. Nos. 3,668,113; 3,894,938; 4,176,050; 4,181,598; 4,222,855; 4,229,282; and 4,247,388. However, the intermediate pore size zeolites such as ZSM-5, which are highly effective as dewaxing catalysts in these hydrogenative processes using relative light feeds, are generally unsuitable for use as cracking catalysts because their pores are too small to admit the bulky, polycyclic aromatics into the internal pore structure of the zeolite where cracking can take place. They have not, therefore, been used as such for catalytic cracking although they have been combined with large pore zeolites in catalytic cracking catalysts in order to improve the octane rating of the naphtha cracking product, but even when combined with a conventional cracking catalyst in this way, they tend to produce too much dry gas and accordingly, they are unable to function effectively as cracking catalysts for waxy feeds. The problem of dealing with feeds of this kind has therefore persisted.